Group III-V compound materials, such as gallium arsenide (GaAs), have higher electron and hole mobility than that of silicon and an excellent carrier transport property, and are expected as next-generation semiconductor channel materials. A group III-V compound channel can be formed by polishing an object including a portion containing a group III-V compound material (hereinafter, also referred to as a group III-V compound material portion) and a portion containing a silicon material (hereinafter, also referred to as a silicon material portion). At this time, in addition to the polishing of the group III-V compound material portion at a high polishing rate, the prevention of the occurrence of a level difference caused by etching is required on a polished surface of the object. However, a polishing composition conventionally used for polishing a group III-V compound semiconductor substrate and described in, for example, Patent Document 1 or 2 is specifically developed for the group III-V compound semiconductor substrate. Therefore, when the polishing composition is used for polishing an object including a group III-V compound material portion and a portion containing a material other than group III-V compounds, the group III-V compound material portion is excessively polished and etched, which makes it difficult to prevent the occurrence of a level difference caused by etching on a polished surface.